The present invention is an improved tobacco smoke filter of the ventilated type which enhances the taste characteristics of the filtered smoke over known ventilation filters.
A ventilated filter may be defined as a filter which is constructed in a manner such that outside air is drawn into the filter for admission, with the tobacco smoke, to the smoker's mouth, in effect diluting the smoke fraction. The ventilated filter makes it possible to use a higher density filtering medium for removal of greater quantities of smoke while not reducing the draw that the smoker expects.
It is well known that the particular method of air dilution affects the taste characteristics of the product. Modifications of an existing filter construction, to vary the manner in which the diluting air is added to the smoke, can have either a positive or a negative effect on the taste. This is demonstrated by the fact that there are a number of known filter designs exhibiting a variety of porous or perforated filter wrap constructions, combined with one of various types of porous or perforated tipping papers, to obtain desirable dilution systems.
In present day cigarette filters, the perforations for admitting diluting air into the smoke mainstream are located over some portion of the filter section, so as to draw air into the filter. Due to the fact that the outside air is drawn in through the filter periphery, it must follow a tortuous route through the filter wrap and filtering material, toward the core of the filter plug, before encountering the smoke mainstream. Rather than flowing into the center of the filter, the diluting air tends to channel along the filter perimeter resulting in an inefficient mixing of air and smoke. The perimetrical air flow pattern also compresses the smoke stream to the core of the filter. As a result, only a portion of the cross-sectional area of the filter is utilized to filter smoke, reducing the filter efficiency.
U.S. Pat. No. 3,490,461 to Osmalov et al. and U.S. Pat. No. 4,256,122 to Johnson illustrate another type of ventilation-type filter. Channels extending along the length of the filter provide a passage for outside air to be drawn, together with the smoke, into the smoker's mouth. Unlike the ventilated filter constructions described above, the filter wrap is impervious and prevents any mixing of the air stream and the smoke stream until such time as the two streams enter the smoker's mouth. The fact that the air and smoke remain separated affects the taste characteristic of the product. Also, the proper functioning of such a design is dependent upon the channels remaining open while the product is being smoked and on the air stream having free access to the mouth. If either of these two factors are changed, there can be a drastic change in the air-to-smoke ratio.
U.S. Pat. No. 3,496,945 to Tomkin discloses yet another construction of a ventilated filter, which employs a flow-restricting plate to maintain a constant air-to-smoke ratio throughout the smoking cycle. Outside air is drawn into the filter assembly, either into a filter plug, or into a chamber, located upstream of the flow-restricting plate, for mixing with the smoke. The air and smoke mixture is drawn through a restrictive flow orifice in the plate and thereafter either through a second filter or directly out through an open end of the mouthpiece.
The Tomkin construction, introducing air upstream of a flow restricting plate, is intended to regulate the flow pattern of the smoke through the filter plug and the flow pattern of the incoming dilution air so as to control the ratio of smoke-to-air. The fact that mixed smoke and air are drawn through the plate orifice will affect the taste characteristics of the product, as well as increasing the draw resistance of the cigarette. The presence of the flow-restricting plate, having only a small orifice for passing the combined flow streams, may also affect the uniformity of flow of the smoke, upstream of the plate, through the filter plug and thus the filtering performance thereof.